Not Applicable
(copyright) 2001 Electro Scientific Industries, Inc. A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR xc2xa71.71(d).
This invention relates to a laser cutting and, in particular, to a method and/or system for advantageous beam positioning and scanning to improve the throughput of laser cutting in silicon or other materials.
FIG. 1 is a simplified representation of a traditional continuous cutting profile 8. Traditional laser cutting employs sequentially overlapping spots from consecutive laser pulses to continuously scan through an entire cut path. Numerous complete passes are performed until the target is severed along the entire cut path. When the target material is thick, many passes (in some cases over 100 passes) may be necessary to complete the cutting process, particularly with limited laser power.
A method for increasing laser cutting throughput for thick materials is, therefore, desirable.
An object of the present invention is, therefore, to provide a method and/or system for improving the throughput for laser cutting silicon or other materials.
For convenience, the term cutting may be used generically to include trenching (cutting that does not penetrate the full depth of a target workpiece) and throughcutting, which includes slicing (often associated with wafer row separation) or dicing (often associated with part singulation from wafer rows). Slicing and dicing may be used interchangeably in the context of this invention.
FIG. 2A is a graph showing that for conventional long continuous throughcuts, the effective dicing speed decreases very quickly as silicon wafer thickness increases. Thus, as thickness increases, the number of laser passes increases almost exponentially and consequently exponentially decreases the dicing speed. The cutting width may be on the order of only a few tens of microns (xcexcm), and the wafer thickness is typically much greater than the cutting width.
Traditional laser cutting profiles may suffer from trench backfill of laser ejected material. When the wafer thickness is increased, this backfill becomes much more severe and may be largely responsible for the dramatic decrease in dicing speed. Moreover, for some materials under many process conditions, the ejected backfill material may be more difficult to remove on subsequent passes than the original target material. Because trench backfill with laser ejected material has a somewhat random nature, the degree of backfill along any portion of a traditional cutting profile may be large or small such that some portions of the cutting path may be cut through (opened) in fewer passes than other portions of the cutting path. Traditional laser cutting techniques ignore these phenomena and continuously scan an entire cut path, including areas that may already be opened, with complete passes of laser output until the target material is severed along the entire cut path.
As an example, a UV laser, having laser output power of only about 4 W at 10 kHz, requires about 150 passes to make a complete cut through a 750 xcexcm-thick silicon wafer using a conventional laser cutting profile. The conventional cutting profiles typically traverse the entire lengths of wafers, which typically have diameters of about 200-305 mm. The resulting cutting rate is too slow for commercial dicing applications of silicon this thick. Although the segmented cutting technique can be employed to cut any laser-receptive material and employed at any laser wavelength, the segmented cutting technique is particularly useful for laser processing at wavelengths where laser power is limited, such as solid-state-generated V, and particularly where such wavelengths provide the best cutting quality for a given material. For example, even though IR lasers tend to provide much more available output power, IR wavelengths tend to crack or otherwise damage silicon, alumina, AlTiC and other ceramic or semiconductor materials. UV is most preferred for cutting a silicon wafer for example.
U.S. patent application Ser. No. 09/803,382 (""382 application) of Fahey et al., describes a UV laser system and a method for separating rows or singulating sliders or other components. These methods include various combinations of laser and saw cutting directed at one or both sides of a wafer and various techniques for edge modification.
U.S. patent application derives priority from U.S. Provisional Application No. 60/297,218, filed Jun. 8, 2001, and is a CIP of U.S. patent application Ser. No. 10/017,497, filed Dec. 14, 2001, which claims priority from U.S. Provisional Application No. 60/265,556, filed Jan. 31, 2001.
FIG. 2B is a graph showing the results of a recent experiment comparing the number of passes to complete a dicing cut versus the cutting length of the cutting profile in 750 xcexcm-thick silicon. A wedge or xe2x80x9cpie slicexe2x80x9d was taken from a 750 xcexcm-thick silicon wafer, and cutting profiles of different lengths were executed from edge to edge. The experiment revealed that shorter cutting profiles could be diced with fewer passes.
The present invention, therefore, separates long cuts into a cutting profile containing small segments that minimize the amount and type of trench backfill. For through cutting or trench cutting in thick silicon, for example, these segments are preferably from about 10 xcexcm to 1 mm, more preferably from about 100 xcexcm to 800 xcexcm, and most preferably from about 200 xcexcm to 500 xcexcm. Generally, the laser beam is scanned within a first short segment for a predetermined number of passes before being moved to and scanned within a second short segment for a predetermined number of passes. The beam spot size, bite size, segment size, and segment overlap can be manipulated to minimize the amount and type of trench backfill. A few scans across the entire cut path can be optionally employed in the process, particularly before and/or after the segment cutting steps, to maximize the throughput and/or improve the cut quality.
The present invention also improves throughput and quality by optionally employing real-time monitoring and selective segment scanning to reduce backfill and overprocessing. The monitoring can eliminate rescanning portions of the cut path where the cut is already completed. In addition, polarization of the laser beam can be correlated with the cutting direction to further enhance throughput. These techniques generate less debris, decrease the heat affected zone (HAZ) surrounding the cutting area or kerf, and produce a better cut quality.
Although the present invention is presented herein only by way of example to silicon wafer cutting, skilled persons will appreciate that the segmented cutting techniques described herein may be employed for cutting a variety of target materials with the same or different types of lasers having similar or different wavelengths.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings.